Additive manufacturing (also known as 3D printing, solid free-form fabrication, rapid prototyping and rapid manufacturing) is commonly used to manufacture three-dimensional solid objects. It is particularly useful for applications where speed of manufacture is important but where low costs are desirable, for example in the manufacture of prototypes.
The additive manufacturing process involves the creation of a three dimensional object by successive addition of multiple material layers, each layer having a finite thickness. A variety of methods fall under the umbrella of additive manufacturing including: stereolithography (SLA), fused deposition modelling (FDM), selective deposition modelling (SDM), laser sintering (LS) and selective light modulation (SLM).
Each of the above known methods includes the following steps:
1. The conversion of a computer-generated 3D model to a file format (such as .STL or .OBJ) which provides geometric information in a physical Cartesian space. Computer aided design (CAD) software may be used to generate the initial 3D model.
2. Once converted, the 3D model is broken down (“sliced”) into a series of two-dimensional (‘2D’) discrete cross sections.
3. A computer controlled apparatus successively fabricates each cross section, one on top of another in the z-direction, forming successive layers of build material on top of another which in turn forms the three dimensional object.
The fabrication process differs between the above-mentioned methods, as does the choice of build material.
The fabrication process used in both stereolithography (SLA) and selective light modulation (SLM) involves a build material of liquid photosensitive polymer (often known as a ‘resin’) and a mechanism for exposing the photosensitive polymer to electromagnetic radiation.
Exposed photosensitive polymer undergoes a chemical reaction leading to polymerization and solidification. The solidification of the photosensitive polymer is commonly known as “curing”, and the solidified photosensitive polymer is said to have been “cured” or “hardened”.
In both SLA and SLM, electromagnetic radiation is applied to a targeted area known as the “working surface”. However, the two processes differ from one another in the way that the electromagnetic radiation is applied to the targeted area: SLA systems use a laser beam mounted on an x-y scanning system to create each material layer of the 3D object by tracing a digital cross-section onto the photosensitive polymer; SLM systems on the other hand, use spatial light modulators such as digital projectors to project the whole digital cross-section onto the photosensitive polymer in one go. The digital projector may be based on: Digital Light Processing (DLP), Digital Micromirror Device (DMD), Liquid Crystal Display (LCD), or Liquid Crystal on Silicon (LCOS).
The apparatus required to carry out SLA or SLM methods usually includes: a vat to hold the photosensitive polymer; a source of electromagnetic radiation (typically UV, near-UV, or visible light); a build platform; an elevator mechanism capable of adjusting the separation of the vat and the build platform; and a controlling computer.
The apparatus may be configured in a “top-down” arrangement in which the source of electromagnetic radiation is located above the vat, or in a “bottom-up” arrangement where the source of electromagnetic radiation is located below the vat.
In a top-down arrangement, such as that shown in FIG. 1A, the source of the electromagnetic radiation is located above the vat. In use, the build platform is positioned below the surface of the photosensitive polymer. The working surface is the photosensitive polymer located above the build platform and the distance between the upper surface of the photosensitive polymer and the upper surface of the build platform defines the cross-sectional thickness of a cured layer. Disadvantages associated with the top-down method include the necessary process of recoating the cured photosensitive polymer with uncured (“fresh”) photosensitive polymer. In addition, the high viscosity of the photopolymer and high surface tension can lead to difficulties in levelling the surface of the photosensitive polymer.
In a bottom-up arrangement, such as that shown in FIG. 1B, the issue of levelling the surface of the photosensitive polymer is avoided by locating the source of electromagnetic radiation below the vat. A layer of photosensitive polymer sandwiched between an optically clear vat floor and the build platform forms the working surface and allows for precise control over the layer thickness and the surface quality of the layer of photopolymer. However, as the photosensitive polymer hardens, it bonds to those surfaces it is in contact with resulting in high separation forces and difficulties in raising the build platform to build the next layer and a risk of damaged to the cured layer.
It is known that damage during separation can be reduced by non-stick coatings and/or thin film layers on the vat. However, these coatings and layers add to the cost of the 3D printing equipment.
Dendukuri, et al (2006), Nature Mater., Vol. 5, pp. 365-369 suggested the application of coatings that inhibit the cure of the photosensitive polymer to the vat floor. A coating of PDMS (an optically clear oxygen rich resin) is applied to the bottom of the vat, the presence of oxygen inhibits the cure of acrylate polymers thus creating a layer of uncured liquid polymer (approximately 2.5μ thick) between the PDMS and the solidified layer. As a result the cured layer does not adhere to the vat floor thus reducing the forces required to raise the elevator. However, when using a cure-inhibition coating, the separation forces between the vat floor and the cured part can be still be very large due to the surface tension forces associated with thin-film viscous liquids. The surface tension forces are particularly important because they are inversely proportional to the layer thickness.
One method of overcoming damage due to surface tension forces is x-translation which utilises a cure-inhibition coating with a slide mechanism and variable depth vat. The cure inhibition coating on the vat floor creates a non-cured layer that acts as a lubricant between the vat floor and the cured part thus the cured part can easily glide on the cure-inhibition layer. The cured cross-section is slid off the cure-inhibition layer into a deeper channel, increasing the distance between the solidified part and the vat floor, reducing surface tension forces by an order of magnitude, allowing the build platform to be raised easily before being moved back to a position above the build platform. This method of translating the build platform from a shallow channel to a deeper channel via translation in the x-direction typically requires an additional “over-lift” step, where the build platform is raised higher than necessary in order to allow for photosensitive polymer to recoat the working surface. Any such additional step/extra movement leads to an undesirable build-up in the time taken to prepare the working surface for the next layer.
As 3D models are sliced into thousands of material layers, it is important to reduce the fabrication time of each cross-section. This depends upon a number of factors such as the time to cure the photosensitive polymer at the desired thickness and the time to prepare the working surface for the next layer. The time to cure the photosensitive polymer is a function of the power of the source of the electromagnetic radiation at the working surface and the composition of the photosensitive polymer. Typically, high power sources result in shorter cure times. The time taken to prepare the working surface for the next layer typically depends on the separation method and time taken to recoat the working surface with fresh photosensitive polymer. Several extra seconds taken during the layer separation process for a model with thousands of layers will add extra hours onto the overall fabrication time.
The apparatus used in the above described SLA and SLM methods tend to be mechanically complex, difficult to operate and maintain and expensive to buy and use. The use of high power lasers and UV light sources tends to significantly increase the cost of the machines both to purchase and to use through high-energy consumption. Furthermore, the health and safety risks of high power laser and UV light source make current systems unsuitable for home use or by untrained personnel.